| O2EM | Magnavox Odyssey2 / Philips Videopac emulator (O2EM), compiled to WebAssembly with a live Intel 8048 debugger. | online |
| Oak | Acorn Archimedes emulator written in modern C++ with SDL, targeting the A3000. | downloadable |
| Octo | Web-based CHIP-8, SUPER-CHIP and XO-CHIP IDE, assembler and emulator by John Earnest. | online |
| Ootake | Actively developed NEC PC Engine / TurboGrafx-16 emulator by Kitao known for very low input lag. | downloadable |
| OpenEmu | macOS multi-system emulator front end integrating over 30 console and computer cores. | downloadable |
| OpenEmulator | High-fidelity Apple II (and other machine) emulator for macOS with authentic monitor emulation. | downloadable |
| Osmose | Open-source Sega Master System and Game Gear emulator originally by Bruno Vedder. | downloadable |
| Out Run | Sega's 1986 dual-68000 arcade racer, running in the browser on a standalone Musashi 68000 core with an in-frame 68000 debugger. | online |
| openMSX | Highly accurate open-source emulator of the full MSX standard. | downloadable |
| oric1js | Pure-JavaScript Oric-1 emulator by Emmanuel Caradec that boots to Oric Extended BASIC in the browser. | online |